Transistors, such as metal oxide semiconductor field-effect transistors (MOSFETs), are the core building block of the vast majority of semiconductor devices. Some semiconductor devices, such as high performance processor devices, can include millions of transistors. For such devices, decreasing transistors size, and thus increasing transistor density, has traditionally been a high priority in the semiconductor manufacturing industry.
A FinFET is a type of transistor that can be fabricated using very small scale processes. FIG. 1 is a simplified perspective view of a FinFET 100, which is formed on a semiconductor wafer substrate 102. A FinFET is named for its use of one or more conductive fins (FinFET 100 includes only one fin 104). As shown in FIG. 1, fin 104 extends between a source region 106 and a drain region 108 of FinFET 100. FinFET 100 includes a gate structure 110 that is wrapped around fin 104. The dimensions of fin 104 wrapped by gate structure 110 determine the effective channel of FinFET 100. FIG. 2 is a simplified perspective view of another FinFET 200; this particular version includes three fins 202 extending between a source region 204 and a drain region 206. As with FinFET 100, a gate structure 208 is formed across the three fins 202. When multiple fins are employed in this manner, it can be extremely important to maintain uniform fin thickness and uniform fin pitch (the distance between two adjacent fins, plus fin thickness).
FinFET devices have historically been formed using silicon-on-insulator (SOI) substrates. Using an SOI substrate, the conductive fins are formed from the silicon material, while the insulator layer provides isolation between adjacent FinFET devices. Bulk silicon substrates are less expensive than SOI substrates, and FinFET devices can also be fabricated using bulk silicon if appropriate isolation methodologies are utilized. One known isolation methodology for FinFETs formed from a bulk silicon substrate requires multiple photolithography and etching steps to create the trenches between n-channel and p-channel transistor devices. The cost and complexity of such multiple photolithography and etching steps can overshadow the benefits of using a bulk silicon substrate rather than SOI.